The present invention relates generally to measurement test instruments for characterizing a transmission medium and more particularly to an improved time domain reflectometer having a low frequency suppression circuit for removing such things as power line influences from time domain reflectometry data.
A time domain reflectometer (TDR) launches interrogating energy pulses from a pulse generator into a transmission medium, such as shielded and unshielded twisted pairs, coaxial cables, and the like, at a given pulse repetition rate, depending upon the designated range of the TDR. During the periods between pulses, acquisition circuitry in a receiver samples the cable to acquire data representative of reflections from flaws, discontinuities, or breaks in the cable. The reflections in the cable are timed from the time of transmission of the energy pulse to determine the range from the transmitter to such flaws, discontinuities, or breaks. Reflections may represent changes in wire gauge, splices, moisture in the cable, and the like. The acquired data is processed and displayed as a waveform trace on a display device, such as a cathode-ray-tube, a liquid crystal display, or the like.
A TDR notes any changes in the characteristic impedance of the cable under test. For a telecommunications copper facility or plant, the characteristic impedance is typically between 100 and 125 ohms. Most unshielded cables fall between 100 and 105 ohms. Shielded cable like T1 is typically about 125 ohms. Any change in the cable's impedance is displayed on the TDR display device as an upward bump, downward dip, or some combination of both deviating from a horizontal trace. The TS100 and the TV110 Time Domain Reflectometers, manufactured and sold by Tektronix, Inc., Wilsonville, Oreg. and assignee of the instant invention, are examples of TDRs for respectively characterizing telephone twisted pair cables and coaxial cable, such as in CATV and the like.
Low frequency signals may be present on the transmission line during cable characterization. A low frequency signal that affects twisted pair cables is a 120 volts, 60 Hz signal induced from adjacent power lines. In coaxial cables cable TV transmission systems, a 120 volts, 60 Hz signal may be applied to the cable conductor of trunk lines for providing power to trunk amplifiers. Existing time domain reflectometers use a high pass filter connected to the input of the receiver circuitry for suppressing these low frequency signals on the transmission medium under test. One example of such a high pass filter, used in the TS100 Time Domain Reflectometer, is a 330 pF capacitor connected to a 10 .OMEGA. resistor. The use of such a high pass filter does not adversely affect the characterization results for shorter cable lengths where the pulsewidth of the interrogating pulses are in nanoseconds. However for longer cable lengths where the pulsewidth is, for example, 3 microseconds the use of the high pass filter does affect the characterization results. The minimum bandwidth requirement for recovering information from a 3 microsecond pulse is in the 300 kHz range. In order to attenuate a 120 volt, 60 Hz signal to a range of about 10 millivolts for characterizing small return reflections requires a high pass filter having a bandpass in the 600 kHz range. Therefore, using a high pass filter for characterizing long cable lengths results in a loss of information.
In the television industry, a DC restorer clamping circuit is used to DC restore or maintain a reference at a predetermined level. Any induced 60 Hz modulation is removed by clamping at each horizontal line interval. The damper is an electronic switch which is usually closed by the occurrence of a horizontal sync pulse, momentarily grounding the signal to earth ground or some fixed DC voltage. The time selected for closure is either the sync tip or the blanking level just after the sync tip.
The operation of this circuit is synchronous with the incoming television signal.
What is needed is a time domain reflectometer that suppresses low frequency signals, such as power line influences, from the receiver of the TDR without limiting the bandwidth of the instrument.